A realistic candidate might be remotely operated vehicle (like those being built by Deep Ocean Engineering), tethered to a base station, he said. "Power would be electrical, propellers, and gyro for guidance. You can't get fancy with deep space missions," French notes, because there's "no one there to fix it."
But it's just for those reasons that underwater gliders are succeeding propeller-driven, electricity-hungry craft to explore and monitor deep-sea conditions here on Earth. These vessels can propel themselves for months using only a trick of buoyancy. Apart from saving energy, these craft also endure better because there are no external moving parts like blades and wing flaps to gum up. Also, should Europa have much particulate matter suspended in its seas, as possibly indicated by the reddish brown colors of the fissures of its ice shell, such an undersea craft wouldn't contaminate its environment with lubricants needed to smooth propeller motion.
"Internal actuation is appealing for applications where a vehicle should not disturb the environment which it is sampling. For underwater vehicles, this might mean avoiding stirring up sediment on the ocean floor, which happens when a vehicle uses external thrusters," said Craig Woolsey, an aerospace and ocean engineer at Virginia Tech developing internal controls suitable for gliders among other applications. No exhaust, no discharge or lubricants. "A space application would involve searching for signs of extra-terrestrial life without jeopardizing it," Woolsey notes.
Underwater gliders work by being buoyantly nearly neutral while near the water's surface, so that they sink slowly down. Very slowly – full steam ahead is under one knot. As they sink, an internal weight is shifted toward the lateral direction the vehicle should go – if you want to go straight, just shift the weight forward to tilt the nose down a bit. Then to rise, pump an oil contained inside the hull into external sacks – this simple maneuver increases the craft's volume, and thus lowers its density just enough that now the surrounding water wants to push it upward.
It's a principle first established by the ancient Greek scientist Archimedes and then adapted for this purpose by the oceanographer Henry Stommel in 1989. And as usual, it seems nature beat human engineers to the punch: marine biologists were for a long time mystified by how dolphins could cross greater distances than their muscle power and oxygen intake should allow. Many researchers attribute this to skin coatings that reduce water friction, but new evidence points to a gliding enabled by buoyancy changes when water pressure compresses air in the respiratory system.
While the Office for Naval Research, which funds much of the glider development, said it hasn't been consulted about extraterrestrial exploration, the topic has already come up in conversations between researchers. Surprisingly, the first target mentioned wasn't a water world, but a sulfuric cauldron.
"Since there are no external actuators, the vehicles are robust to corrosion. A colleague, Dr. Chris Hall, suggested to me the possibility of using gliders in caustic extra-terrestrial atmospheres, such as that of Venus" explains Woolsey, a recipient of ONR funding. Venus, unlike Mars, has a thick and soupy atmosphere that could support the weight of a glider for an extended time, much like water.
Though Woolsey said weight displacement (changing the center of gravity) as a means of steering is a robust and efficient system; he's researching another method that might offer distinct advantages for scientists wanting to get close and steady images of life forms or geological features. Instead of weights, he's designing rotors that by spinning change the moment of inertia, the angular momentum, of a craft. That would allow for delicate moves – small rotations to keep a site in view, for example – that free weights might not be as capable of executing.
"You could turn on a dime with a rotor. You're also not blowing your subject out of the field of view," Woolsey said.
The University of Washington has already done extensive and successful testing on its Seaglider probes. The small machines have had "flights" lasting a month, and "the power supply in the current version should allow 6 months of flight, which gives it a range of about 5000 km with average ocean currents. If the present batteries were replaced with higher capacity versions, the lifetime could be well over a year. Of course, a larger vehicle, which would allow a larger batter pack, could last much longer," said Robert C. Spindel, director of the Applied Physics Laboratory of the University of Washington.
Webb Research Corporation offers an even more radical approach. It requires no electricity at all for locomotion. The Slocum deep-sea probe harnesses temperature gradients for its power.
"You're harvesting propulsion energy from its environment – heat flow from warm place to a cold place is capable doing of doing work. You're not violating any laws of thermodynamics," explained Douglas Webb, the company's founder. "The goal is finding neat and graceful ways to do it."
Inside Webb's craft is a cavity filled with an organic liquid compound that contracts when cooled and expands when warmed. When the Slocum rises to the surface, the warming sun expands the material, pushing a spring. That spring stores energy as tension and when the vessel dives deep, the water cools. When the spring is released, it pushes the flotation oil out into the external bladders to achieve buoyancy. When the Slocum reaches the surface, the process repeats itself.
On Venus or Europa, that process is somewhat reversed. The gravitational flexing of Europa's metallic core warms the moon form within. Some planetologists suspect it may have hot vents or volcanoes at the bottom of its ocean. Venus is also cooler in its upper atmosphere and quite hot near the surface because of its heat trapping clouds.
"A reversed gradient is nothing I would be too concerned about. By having stored the energy in this accumulator (spring) I can get the work done at any part cycle. As long as we have a heat flow, there is potential for doing work," Webb said. "What I can't work in is an isothermal (evenly distributed heat) environment."
That would mean that gliders in the methane and ethane oceans of Titan, another world on the astrobiology short list, would require batteries. But otherwise, some of the same methods should work – Titan has significant gravity. It's seas, though not water, should be a near match because liquid methane is only slightly less dense than water, Jack Jones, Task Manager for Planetary Balloon Buoyancy Systems at NASA explained. He is designing balloons for Venus (Teflon covered), Titan's atmosphere, and Mars. "Gliders and airplanes operating in the thin martian atmosphere can function for only some tens of minutes before crashing into the surface," he noted.
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